Even in the absense of alignment (which means that it generally takes up a minimum of 16 bytes of real memory), it takes up what would tend to be a much more critical resource: a cache line.

So an empty macro is a _lot_ more efficient than an empty function call.

For interrupt handlers, the L1 I$ miss rate is basically 100%. Even the L2 I$ miss rate is quite noticeable, _especially_ for things that are not easy to prefetch (ie stuff that is out-of-line). Which means that it also likely takes an inordinate amount of cycles, for doing zero work.